Presently, an optical fiber used for an optical fiber communication system spreading generally has a structure in which outer circumference of one core is surrounded by a clad and an optical signal is propagated through the core, so that information is transmitted. Recently, an amount of information transmitted increases drastically with the spread of the optical fiber communication system. According to the increase in the amount of information transmitted, in the optical fiber communication system, a large number of optical fibers such as dozens or hundreds of optical fibers are used and large-capacity long-distance optical communication is performed.
It is known that, in the optical fiber communication system, a multi-core fiber in which outer circumferences of a plurality of cores are surrounded by one clad is used and a plurality of signals are transmitted using light propagated through the individual cores.
An example of the multi-core fiber is described in Patent Literature 1 described below. In the multi-core fiber, one core is disposed on a center of a clad and six cores are disposed around the core disposed on the center. Because such an arrangement is a structure in which the cores can be disposed in a closest packing state, a large number of cores can be disposed with respect to an outer diameter of a specific clad. In addition, in the multi-core fiber described in Patent Literature 1, propagation constants of light propagated through the cores adjacent to each other are different from each other to suppress crosstalk of light propagated through the individual cores.
However, there is a request for suppressing the crosstalk more than the case in which effective refractive indexes of the cores adjacent to each other are changed like the multi-core fiber described in Patent Literature 1. Therefore, a multi-core fiber in which a low refractive index layer having a refractive index lower than a refractive index of the clad is disposed to surround an outer circumferential surface of each core and the crosstalk is further prevented is known. This multi-core fiber is described in Patent Literature 2 described below. When the multi-core fiber is viewed from a viewpoint of the refractive index, the low refractive index layer has a shape of a trench. For this reason, the multi-core fiber is called a trench type and a configuration from the core to the low refractive index layer is called a core element. Even in the trench type multi-core fiber, propagation constants of light propagated through the cores adjacent to each other are preferably different from each other to suppress the crosstalk of light propagated through the individual cores.
[Patent Literature 1] JP2011-170336 A
[Patent Literature 2] JP2012-118495 A
However, it is necessary to change refractive indexes or diameters of the cores adjacent to each other to change the propagation constants of the light propagated through the cores adjacent to each other as described above. When communication is performed using light of a desired mode in a desired wavelength band, a range of obtained values of the refractive indexes or the diameters of the cores is narrow and there is a limitation in a degree of freedom for design to change the refractive indexes or the diameters of the cores adjacent to each other.
In addition, in the trench type multi-core fiber, there is a tendency that it is hard to escape light of a higher-order mode in light propagated through a specific core or core element and a cutoff wavelength increases, when the core element is disposed to surround the specific core or core element. Therefore, to suppress propagation of light of a higher-order mode as compared with a mode of light propagated when the core element exists alone, a core pitch cannot be decreased greatly and there is a limitation in a degree of freedom for design.